The dependence of erectile behavior on androgen functioning is well established. Castration produces loss of both libido and potency in man and animals. The present study, using an animal model for potency, demonstrates the dependence of centrally induced erectile behavior on an intact androgen milieu. Castrated rats failed to produce an erection in response to apomorphine, an agent shown to produce erection in nearly all normal rats. Administration of exogenous testosterone propionate in dosages exceeding 60 micrograms./kg. produced a significant increase in erectile behavior. Yawning, an essentially parallel phenomenon to the stimulation of the erectile response, was also decreased following castration and responded similarly to increasing amounts of exogenous testosterone, demonstrating the influence of androgen functioning on the central nervous system. It was concluded that testosterone is a necessary prerequisite for the maintenance of a centrally induced erectile and yawning response. In an animal model of penile erection, testosterone increases the number of erections in a dose-dependent manner in castrated rats. The dependence of the erectile response on testosterone is, at least in part, centrally mediated.